1. Technical Field of the Invention
The invention relates to a drive for an electrically-actuatable vehicle brake.
2. Prior Art
DE 195 43 098 C2 describes an electrically-actuatable vehicle brake. In this known brake system, the electrically-actuatable vehicle brake is embodied as a disk brake whose brake shoes can be pressed against the brake disk with the aid of an electric motor serving as an electric drive. Moreover, by way of a spindle drive in the form of a planetary-roller threaded spindle known from DE 37 39 059 and disposed in the direction of displacement of the frictional elements, the electric motor is connected to a piston that is seated to be axially displaceable and acts on the brake shoe.
This known planetary-roller threaded spindle has low frictional coefficients, and permits a high force transmission, so the force required for actuating vehicle brakes can only be attained in one gear stage. Due to its function principle, however, a disadvantage of this spindle drive is that the transmission of the tangential forces from the spindle shaft to barrel or roller bodies and from these bodies to the spindle shaft is affected by slip. Thus, the known planetary-roller threaded spindle has an imprecisely-defined pitch and, consequently, no constant force transmission.
For operating a brake, however, the force with which brake shoes are pressed against a brake disk must be known. If a spindle gear is used that is not pitch-true, neither the position nor the speed nor the force acting on the brake lining can be determined from the motor parameters such as current, torque, rpm and motor setting. Consequently, to determine such values as are necessary for a control/regulation, additional sensors and evaluation units are required, which are costly on the one hand and, on the other hand, are associated with additional, complicated technology. Furthermore, the sensors in the region near the brake disk require space that is severely reduced there. Also, due to the proximity to the brake disk, high temperatures occur. The sensors would therefore have to be heat-resistant and free from temperature drift, or thermally shielded. This is, however, associated with complex technology and a high financial outlay.
Moreover, a few gear-related solutions have already been proposed for constructing a compact, electrically-actuatable brake actuator. None of the heretofore known proposals has succeeded in generating the high forces, with a constant pitch, that are required for actuating a brake from the rotational movement of the drive motor in the form of an electric motor.